floral symmetry

...Differences in size or shape of the parts of a whorl make the flower irregular (as in the canna and Asiatic dayflower). When a flower can be divided by a single plane into two equal parts, it is zygomorphic, or bilaterally symmetrical, as in the snapdragon, orchid, and sweet pea.

...any line drawn through the centre will divide the flower into two identical halves. When at least one petal of the corolla is different, the flower has bilateral symmetry and is called irregular or zygomorphic (e.g., violets,
Viola; Violaceae).

Papilionoideae

...tribes. The name of the group probably originated because of the flower’s resemblance to a butterfly (Latin:
papilio). It is the unique bilaterally symmetric (zygomorphic) flowers that especially characterize the group, so that thousands of species can be recognized as a member of Papilionoideae at a glance. The
Lathyrus odoratus (sweet pea) flower...